The present invention relates generally to a method and an apparatus for controlling a hydraulically-operated continuously variable transmission of belt-and-pulley type. More particularly, the invention is concerned with such controlling method and apparatus which permit a change in speed ratio of the transmission with improved response.
A continuously variable transmission of belt-and-pulley type is known, which comprises an input shaft operatively connected to a drive source such as an engine of a vehicle, an output shaft disposed parallel to the input shaft, a first variable-diameter pulley including a pair of opposed rotary members mounted on the input shaft, a second variable-diameter pulley including another pair of opposed rotary members mounted on the output shaft, and a transmission belt connecting the first and second variable-diameter pulleys. The effective diameter of each variable-diameter pulley is changed by axial movements of the rotary members toward and away from each other by a hydraulic cylinder. In a generally used hydraulic arrangement of such a transmission of belt-and-pulley type, one of the first and second hydraulic cylinders for changing the effective diameter of the first and second variable-diameter pulleys is always fed with a predetermined flow of pressurized fluid supplied from a suitable hydraulic pressure source (pressure generating device). In the meantime, the hydraulic flow to be supplied to or discharged from the other hydraulic cylinder is varied so as to change the effective diameter of the associated pulley. For example, the first hydraulic cylinder is used to positively change the effective-diameter of the first pulley, while the second hydraulic cylinder is fed with a hydraulic line pressure just sufficient to give a suitable tension to the transmission belt so that the effective diameter of the second pulley is changed following the changing effective diameter of the first pulley, whereby the speed ratio between the input and output shafts of the transmission is continuously variable. When this type of variable transmission is used for a vehicle, the speed ratio of the transmission is changed according to changing running conditions of the vehicle, so that the fuel economy of the vehicle is improved.
In a known controlling apparatus for such a variable transmission, there is generally used a valve device which has three states: a first state at which the predetermined hydraulic pressure is supplied to the first hydraulic cylinder; a second state at which the hydraulic pressure is discharged from the first hydraulic cylinder; and a third state at which the supply and discharge of the hydraulic pressure to and from the first hydraulic cylinder are restrained. In this arrangement, the speed ratio of the transmission is varied at a fixed rate of shifting. Accordingly, it is not possible to control the transmission such that the actual speed ratio is varied with a satisfactory response to a change in a target speed ratio which is continuously determined to establish running conditions of the vehicle desired by the driver, with mximum fuel economy. The controlling apparatus may suffer a delayed response of the actual speed ratio to the target speed ratio, or a hunting trouble, particularly when the desired vehicle speed is abruptly changed.
Stated in more detail, the known controlling arrangement for a variable transmission of belt-and-pulley type is not completely satisfactory in its operating reliability and accuracy in controlling a flow of a pressurized working fluid for changing the speed ratio of the transmission. For example, a controlling arrangement in the prior art is adapted to control the actual speed of a vehicle engine so as to coincide with a target engine speed which is determined based on an opening angle of a throttle valve (which represents a currently required output of the engine). In such an arrangement, the actual engine speed tends to be fluctuated in the vicinity of the determined target engine speed, i.e., a hunting or overshooting trouble will take place in the process of controlling the actual engine speed in conformity with the target speed. This trouble will cause undesired oscillation or vibration of the engine, which may possibly degrade the drivability of the vehicle.
It is possible that the valve device used in the controlling apparatus described above be replaced by a proportioning flow control valve which is capable of continuously changing its opening angle for continuous change of a fluid flow through the valve. However, such a flow control valve inherently suffers a variation in the flow to be controlled, due to presence of foreign matters in the working fluid or due to inaccuracy of manufacture of the valve. Thus, a sufficiently accurate control of the variable transmission is not necessarily obtained by such a flow control valve.